This invention generally relates to a shape memory alloy and, in particular, to an NiMnGa magnetic alloy having a shape memory effect.
In general, it is known that a shape memory alloy, such as a TiNi alloy or a CuZn alloy, exhibits a remarkable shape memory effect and a superelasticity.
Such an alloy has an austenite phase at a relatively high temperature and a martensite phase at a relatively low temperature. Upon the temperature drop of the alloy from the relatively high temperature to the relatively low temperature, the alloy phase transforms or transforms from the austenite phase to the martensite phase. The phase transformation is called the martensitic transformation. On the other hand, the other reverse phase transformation from the martensite phase to the austenite phase accompanied with temperature elevation is referred to as an austenitic transformation. Since the austenitic transformation is the reverse transformation of the martensitic transformation and, it is often referred to as the reverse transformation.
Providing that the alloy is formed into a shape as an original shape at the austenite phase and then cooled without deformation of the original shape into the martensite phase, the alloy is deformed from the original shape into a desired shape at the martensite phase. Thereafter, when the alloy is exposed to a temperature elevation and transformed to the austenite phase, the alloy changes in shape from the desired shape into the original shape. The alloy has a shape recovery effect by the temperature elevation or the reverse transformation. This means that the alloy memorises the original shape. That is, the alloy has the shape memory effect.
On the temperature axis for the both phase transformation, the alloy has a start point and a finish point of the martensitic transformation which will be referred to as Ms point and Mf point, respectively, and also a start point and a finish point of the austenitic or reverse transformation which will be referred to as As point and Af point, respectively. Both transformation have a hysteresis on the temperature axis, and therefore, Ms point and Af point are not coincident with but different from each other, and Mf point and As point are not coincident with but different from each other, too.
The shape memory alloy as well as other metal has usually elasticity against a deformation or strain under a limited stress or strain which will be known as a yield point. A particular one of the shape memory alloy has a nature where it exhibits a large strain suddenly after exceeding the yield point and recovers from the strain to the original non-strain condition when the stress is unloaded. This nature is referred to as the super-elasticity. The superelasticity is usually present around the Af point or just above the Af point.
Among others, the TiNi alloy is known as an alloy having the most excellent shape memory effect and is widely used, for example, as temperature responsive actuators in a ventilator of a house, an air conditioner, a rice cooker, and a shower valve. The TiNi alloy has also excellent superelasticity and is used for an eyeglass frame, medical instruments such as a catheter, and an antenna of a mobile telephone.
On the other hand, an Ni2MnGa alloy is known as a magnetic alloy which has the martensitic transformation and the reverse transformation along the temperature drop and elevation, respectively. According to the martensitic and reverse transformation, the Ni2MnGa alloy is known to change in magnetism. That is, it is changed from paramagnetism into ferromagnetism at the Af point upon the reverse transformation from a low temperature phase into a Heusler type high temperature phase by temperature elevation. The Af point Ni2MnGa alloy is about xe2x88x9250xc2x0 C. It should be noted that the Af point is different from the Curie point which is known as a point where the alloy changes in the magnetism from the ferromagnetism to the paramagnetism upon the further temperature elevation. Therefore, Ni2MnGa alloy exhibits the ferromagnetism within the temperature range between the Af point and the Curie point Tc but is paramagnetism in the other temperature region. The Curie point of the Ni2MnGa alloy is about 105xc2x0 C. In the present status, however, no technique has been found out to shift or control the Af point. Thus, it is impossible to use the Ni2MnGa alloy as functional elements such as temperature responsive magnetic elements which is operable around a normal living environment temperature, for example, xe2x88x9220xc2x0 C. to +50xc2x0 C.
Further, the Ni2MnGa alloy was believed to have no shape memory effect.
It is an object of this invention to provide an NiMnGa alloy which has a finish point (Af) of the reverse transformation of the martensitic transformation around a normal living environment temperature and which is therefore applicable to temperature responsive elements.
According to this invention, there is provided an NiMnGa alloy represented by a chemical formula of Ni2+XMn1xe2x88x92XGa (0.10xe2x89xa6Xxe2x89xa60.30 in mol) and having a finish point of the reverse transformation of the martensitic transformation at a temperature equal to xe2x88x9220xc2x0 C. or more.
According to an aspect of this invention, the finish point can be selected at a temperature within a range between xe2x88x9220xc2x0 C. and 50xc2x0 C. with the Curie point at a temperature within a range between 60xc2x0 C. and 85xc2x0 C.
According to another aspect of this invention, there is also provided an NiMnGa alloy which has the shape memory effect accompanied with the martensitic transformation and the reverse transformation along the temperature variation.
According to another aspect of this invention, there is also provided an NiMnGa alloy which has a characteristic wherein the reverse transformation is induced by application of an external magnetic field at a condition of the martensite phase, to thereby cause a shape recovery.
Now, description will be made in detail as regards an NiMnGa alloy of this invention in conjunction with specific examples thereof.
At first, an outline of the NiMnGa alloy of this invention will be briefly described. This invention is based on the findings by the present inventors that, in the NiMnGa alloy, the finish point (Af) of the reverse transformation can be shifted or controlled at a temperature within a predetermined range by changing composition ratio of Ni and Mn. The present inventors have also found out that the NiMnGa alloy exhibited the shape memory effect accompanied with the martensitic transformation and the reverse transformation.
Specifically, the NiMnGa alloy of this invention is characterized as follows. In the NiMnGa alloy represented by the chemical formula of Ni2+XMn1xe2x88x92XGa, a composition ratio parameter X (mol) is selected within the range of 0.10xe2x89xa6Xxe2x89xa60.30. With this composition, the finish point Af of the reverse transformation can be selected to a desired temperature within the range between xe2x88x9220xc2x0 C. and 50xc2x0 C. while the Curie point Tc being selected to a desired temperature within the range between 60xc2x0 C. and 85xc2x0 C. . Furthermore, it has been found out that the reverse transformation of martensitic transformation can be induced by application of an external magnetic field to the Ni2+XMn1xe2x88x92XGa alloy and the shape recovery can thereby be performed.
Therefore, the NiMnGa alloy-according to this invention can be expected to be used onto various applications such as temperature and/or magnetic responsive elements under the normal living environment.